Cathode side hardware for carbonate fuel cells

ABSTRACT

Carbonate fuel cathode side hardware having a thin coating of a conductive ceramic formed from one of LSC (La 0.8 Sr 0.2 CoO 3 ) and lithiated NiO (Li x NiO, where x is 0.1 to 1).

BACKGROUND OF THE INVENTION

This invention relates to molten carbonate fuel cells and, inparticular, to cathode side hardware employed in such cells.

As used herein the term “cathode side hardware” is defined as thecurrent collector and/or the bipolar plate on the cathode side of a fuelcell and, in particular, a molten carbonate fuel cell. Corrosion is alife-limiting factor for a molten carbonate fuel cells. The prevailingcorrosion is at the oxide-gas (or liquid) interface, i.e., at thecathode side hardware. This hardware is typically formed from chromiumcontaining stainless steel and corrosion of the hardware is governed bythe outward cation diffusion via metal vacancies. It is estimated thattwenty five percent (25%) of the internal resistance of a moltencarbonate fuel cell could be attributed to the oxide corrosion layerthat forms on the cathode side hardware.

More particularly, the cathode current collector, generally made of 316Lstainless steel, becomes corroded during fuel cell operation andmulti-corrosion oxide layers having a relatively high electricalresistance are formed on the surface of the collector. Moreover, theformed corrosion layers usually thicken with time.

Additionally, the corrosion layers on the cathode side hardware causeelectrolyte loss through surface and corrosion creepage. Electrolytesurface creepage is controlled by capillary forces dominated by thesurface roughness, porosity and pore size in corrosion layers.Electrolyte corrosion creepage is controlled by scale thickness andphase composition of the formed scale. In cathode side hardware formedwith stainless steel, a high roughness of the scale surface and theporous structure of the scale cause high electrolyte surface creepage.

It has been estimated that electrolyte loss in a molten carbonate fuelcell is a significant life-limiting factor for achieving a lifetime of40,000 hours. Analysis of cathode side hardware has indicated that sixtyfive percent (65%) of electrolyte loss is attributed to this hardware.It is estimated that a forty five percent (45%) reduction in electrolyteloss could result in ˜1.7 yr life extension of the molten carbonate fuelcell.

In order to counter the corrosion of the cathode side hardware, it hasbeen proposed to provide a protective oxide coating on the cathode sidehardware to realize a low contact resistance and low electrolyte loss.These coatings, however, must satisfy stringent requirements in thatthey must, on the one hand, have a high corrosion resistance, and, onthe other hand, a high electrical conductivity. The coatings must alsobe able to provide a stable surface oxide capable of providing a barrierbetween the coating alloys and the environment of the molten carbonatefuel cell.

U.S. Pat. No. 5,643,690 discloses a coating of this type in the form ofa non-stoichiometric composite oxide layer (Ni ferrite based oxide)formed by in cell oxidation of a layer of Fe, Ni and Cr clad on cathodecurrent collector. Similarly Japanese patent 5-324460 discloses astainless steel collector plate covered with a NiO layer (formed byoxidation of a Ni layer plated or clad on a cathode current collector).The coatings formed in these cases are porous and consume a significantamount of electrolyte. Also, the electrical conductivity of the layersmay not be as high as desired.

U.S. patent application Ser. No. 10/016,552, assigned to the sameassignee hereof, discloses another coating layer which is formed as aconductive layer of ceramic material using a sol-gel process. Thematerials used for the conductive layer in this case are, preferably,LiCoO₂ or Co doped LiFeO₂, and the thickness of the layer is between 1to 5 μm.

The aforesaid conductive ceramic layers of the '552 application haveproven satisfactory in providing corrosion resistance of the cathodeside hardware. However, the materials are costly and add to the overallexpense of the fuel cell. Moreover, higher conductivities are stilldesired. Fuel cell designers have thus continued to search for othercoating materials which offer the desired corrosion resistance, but aremore cost effective and are higher in conductivity.

It is therefore an object of the present invention to provide cathodeside hardware which does not suffer from the above disadvantages; and

-   -   It is a further object of the present invention to provide        cathode side hardware having a high corrosion resistance and        electrical conductivity and a lower cost.

SUMMARY OF THE INVENTION

The above and other objects are realized in cathode side hardware byforming the hardware to have a thin film of a dense conductive ceramiccoating comprised of LSC (La_(0.8)Sr_(0.2)CoO₃) or lithiated NiO(Li_(x)NiO, where x is 0.1 to 1). Preferably, the coating is realizedusing a sol-gel process.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and aspects of the present invention willbecome more apparent upon reading the following detailed description inconjunction with the accompanying drawings, in which:

FIG. 1 schematically illustrates a fuel cell including cathode sidehardware in accordance with the principles of the present invention;

FIGS. 2A and 2B show SEM micrographs of different magnifications of aconductive lithiated NiO coating of cathode side hardware in accordancewith the principles of the present invention;

FIG. 3 shows an SEM micrograph of a conductive LSC coating of cathodeside hardware in accordance with the principles of the presentinvention;

FIG. 4 shows the phase evolution with heat treatment temperature of alithiated NiO coating of the type shown in FIGS. 2A and 2B synthesizedby a sol-gel process;

FIGS. 5A-5D illustrate the effect of immersion corrosion testing both oncathode side hardware coated with a lithiated NiO conductive coating inaccordance with the invention and on uncoated cathode side hardware.

FIGS. 6A and 6B show the out-of-cell electrical resistivity and theout-of-cell metal-to-metal electrical resistivity, respectively, ofuncoated cathode side hardware and cathode side hardware coated with alithiated NiO coating and an LSC coating in accordance with theinvention;

FIGS. 7 and 8 show the resistance lifegraphs of molten carbonate fuelcells having cathode side hardware with the lithiated NiO and LSCconductive ceramic coatings of the invention and fuel cells withuncoated cathode side hardware;

FIG. 9 shows the corrosion thickness after fuel cell testing of cathodeside hardware using the conductive ceramic coating of the invention ascompared to the corrosion thickness after fuel cell use of uncoatedcathode side hardware; and

FIG. 10 illustrates the electrolyte loss in a molten carbonate fuel cellusing cathode side hardware having the conductive ceramic coating of theinvention as compared to the electrolyte loss in a molten carbonate fuelcell using uncoated cathode side hardware.

DETAILED DESCRIPTION

FIG. 1 schematically shows a fuel having a cathode 2 and an anode 3.Between the cathode 2 and the anode 3 is a matrix 4 containing an alkalicarbonate electrolyte. Adjacent the anode 3 is a corrugated currentcollector 3 a and a bipolar plate 3 b. Adjacent the cathode 2 is thecathode side hardware 5 comprising a corrugated current collector 5 aand a bipolar plate 5 b. As shown, the bipolar plates 3 b and 5 b arethe same.

In accordance with the principles of the present, cathode side hardware5 of the fuel cell 1 of FIG. 1 is coated with a conductive ceramic toobtain lower electrical resistivity for lower contact voltage loss. Infurther accord with the invention, LSC (La_(0.8)Sr_(0.2)CoO₃) andlithiated NiO (Li_(x)NiO, where x is 0.1 to 1), which have previouslybeen proposed for use as cathode materials, are now utilized as coatingmaterials for the cathode side hardware. These materials have a lowsolubility in alkali molten carbonate (e.g., 2 μg/cm²/h for lithiatedNiO) and very high electrical conductivity (600 S/m at 600° C. for LSC,33 S/cm at 650° for lithiated NiO).

These materials can be coated on the cathode side hardware using avariety of coating techniques. In the present illustrative case, a thinfilm sol-gel coating process has been employed. This process involvesthe dissolution of precursors containing the required metal ions in asuitable solvent to form the sol. The sol is then coated on the hardwaresurface by a spray or dipping process, subsequently gelled, and dried,followed by densification and crystallization.

Drying is generally performed between room temperature and 200° C. Thedensification and recrystallization processes are usually carried out attemperatures above 350° C. The surface of the metal substrate mayrequire degreasing and pickling to remove surface debris and oxide forbetter coating adhesion. Although 100% of coating coverage is notnecessary for carbonate fuel cell application in terms of ohmic contactresistance, it is desirable to have >95% coverage of the surface by theceramic coating to achieve the desired benefits of increased corrosionprotection and reduced electrolyte loss. The resultant cathode sidehardware can thus be provided with the required structure and phaseassemblage to provide the desired properties.

The precursors for LSC (La_(0.8)Sr_(0.2)CoO₃) and lithiated NiO(Li_(x)NiO, where x is 0.1 to 1) can be acetates or inorganic salts likenitrate or hydroxide. The solutions can be aqueous based or solventbased. The body or substrate member of the cathode side hardware can bestainless steel.

By properly controlling processing parameters, the coating technique andheat treatment (sol-gel in this case), a dense, uniform and smooth thincoating can be obtained. The major corrosion resistance of the denseoxide coated cathode side hardware of the invention is governed bytransport through the purposefully coated dense oxide layer. The denseoxide coating significantly delays mobile carbonate ion attack of theunderlying body (e.g., stainless steel body) of the hardware.

In particular, the main effect of the formed oxide layer is to barriergas, vapor and liquid contact with the hardware body. On the other hand,due to the corrosion resistant oxide layer being highly conductive, thecontact resistance between the hardware body and the cathode electrodeis also reduced as compared with corrosion scale formed on hardwarewhich is uncoated. In comparison to an uncoated hardware body formed ofchromium steel, the electrical resistance is lowered 50% as exhibited inout of cell testing.

Moreover, due to the dense and smooth surface of the LSC and lithiatedNiO coatings of the cathode side hardware of the invention, surfaceroughness and corrosion are both minimized. Accordingly, electrolytesurface and corrosion creepage are also minimized.

The coatings of the cathode side hardware have also been found toexhibit favorable adhesion, well matched thermal expansion coefficients,effective electronic conductivity, and protection against hotoxidation/corrosion. Accelerated thermal cyclic testing of the coatingshave indicated that the coatings are thin and are very adhesive. Goodadhesion is attributed to reaction-bonded structure between the coatingand the body of the hardware and also its thin film character (tensilestress is proportional to coating thickness). Matching of bulkproperties over the harsh temperature and chemical potential ranges ofthe fuel cell have also been achieved.

As above-noted, the coatings of the invention (Li_(0.1)NiO andLa_(0.8)Sr_(0.2)CoO₃) also exhibit a much higher conductivity (33 S/cmof Li_(0.1)NiO and 650 S/cm of LSC) and more importantly, are dense andsmooth with a controlled coating thickness, as compared with the LiCoO2coating (conductivity of 1 S/cm) of the '522 application.

The following are two examples of the invention.

EXAMPLE I Lithiated NiO Cathode Side Hardware

(1) Preparation of Lithiated NiO Sol-Gel A starting solution with anominal Li:Ni composition of 0.1:1 (molar ratio) was prepared usingreagent grade Li acetate and Ni acetate as cation source compounds.Appropriate quantities of these materials to be included in the startingsolution were then calculated on the basis of obtaining 1 M NiO sol-gel.Measured amounts of the cation source compounds were then mixed with 200ml distilled water, 300 ml ethylene glycol and 1.5 mol citric acid in a600 ml beaker to form a precipitate-free starting solution. The startingsolution was heated on a hot plate at about 80° C. to concentrate untilit turned to a viscous liquid. A green, transparent solution resulted.The solution was allowed to stand at 25° C. in a sealed glass beaker forat least half a year without precipitation. The change in the viscosityof the solution due to polymerization was measured at room temperatureby means of a Brookfield viscometer. The viscosity of the precursorincreases significantly with increased heating time due to the increasein average molecular weight as a result of polymerization.

(2) Deposition and Formation of Cathode Side Hardware With the Dense,Smooth Lithiated NiO Films

A dip-coating technique was used to form wet films of the precursor oncorrugated stainless steel (316L) sheets or substrates used as thecathode side bodies. The film thickness was established by controllingwithdrawal speed and the viscosity of the precursor. In general,precursors with viscosity below 125 cp at room temperature could nothomogeneously wet a smooth substrate, such as stainless steel. On theother hand, highly viscous precursors having a viscosity above about1000 cP at room temperature resulted in inhomogeneous films and crackformation unless the substrate was heated at higher temperatures.Therefore, it is important to control the viscosity of the solution toobtain high quality films. The viscosity of the precursor solutions usedin this Example ranged between 200 and 275 cP at room temperature. Witha withdrawal speed of 1 inch/min, dense lithiated NiO oxide films wereobtained having a thickness in the range of about 0.5 to about 1 micronfor each coating after firing at about 600° C. To increase sol wettingand increase bond strength between the lithiated nickel oxide film andstainless steel sheets, the sheets were acid treated first, followed byacetone washed ultrasonically to remove any possible dusts and carbonfilm formed during heat treatment in graphite furnace. Gel film weredeposited in a either 1.5 inch by 1.5 inch or 7 inch by 7 inch of acathode corrugation by dip coating at a substrate withdrawal rate of 1inch/min. Substrates with gel films were transferred to a furnacepreheated to 70° C. just after gel film deposition, held for 3 hours andheated up to a temperature no more than 600° C. for another 3 hours forcomplete crystallization and sintering.

After heating to 600° C., the film remained continuous, smooth and denseon the cathode side hardware body. FIGS. 2A and 2B show SEM photographsof a fractured cross section of the film coated hardware at differentmagnifications. From these figures, it can be seen that a thin lithiatedNi oxide film of substantially uniform thickness ˜1 micron, wasrealized.

(3) Characterization of the Lithiated NiO films

The phase evolution of the Li_(0.1)NiO oxide films deposited on thestainless steel (316L) substrates were studied by x-ray diffractionanalysis performed on a Philips diffractometer, using Cu Ka radiation.The result is shown in FIG. 4. The substrate heat treated at 200° C.shows no peaks, indicating amorphous structure. The formation of therock salt NiO structure is observed at temperatures as low as 350° C.However, there is a secondary phase formed at 350° C. and is indexed asNiC. At higher temperature, NiC oxidizes to form NiO. At 450° C.,crystallization is complete, there is no broadened peaks and theintensities of peaks did not change above 450° C. Except for 200° C. and350° C., all the coatings are single phase, neither other secondaryproduct nor reactant was detected.

As we can see, lithiated NiO adopt a NiO (rock salt structure) structureand the patterns can be indexed in the Braggs peaks of NiO. There is nopreferential position for lithium and nickel ions; thus, there is onlyone cation crystallographic site. The partial substitution of Nickel(II) (r=0.83 Å) by lithium (I) (r=0.90 Å) implies the creation of nickel(III) (r=0.70 Å) whose ionic radius r is smaller, which shrink thelattice. With higher heat treatment temperature, phases shift to largerangles implying more Li dissolving in the NiO structure (more Ni (III)is created). Also peaks become narrower with increasing temperatureindicating crystallite size growth.

EXAMPLE II La_(0.8)Sr_(0.2)CoO₃ Cathode Side Hardware

(1) Preparation of La_(0.8)Sr_(0.2)CoO₃ Sol-Gel

A starting solution with a nominal La:Sr: Co composition of 0.8:0.2:1(molar ratio) was prepared using reagent grade La nitrate hydrate and Srnitrate hydrate and Co nitrate hydrate. Appropriate quantities of thesematerials to be included in the starting solution were then calculatedon the basis of obtaining 1 M LSC sol-gel. Measured quantities of thecation source compounds were then mixed with distilled water (150 ml),ethylene glycol (350 ml) in a 600 ml beaker to form a precipitate-freestarting solution. The starting solution was then heated on a hot plateat about 80° C. to expel the water and other volatile matter and form aviscous polymer precursor comprising a polymer containing the metalcations. It is important that the cations remain in solution throughoutthe polymerization process. The change in the viscosity of the solutionas it was converted into the polymeric precursor was measured at roomtemperature by means of a Brookfield viscometer.

(2) Deposition and Formation of Cathode Side Hardware With Smooth, DenseLSC Films To increase sol wetting and increase bond strength between theLSC film and the stainless steel (316L) cathode side hardware, thestainless steel was acid treated first, followed by acetone washedultrasonically to remove any possible dust and carbon film formed duringheat treatment in graphite furnace. Gel film were deposited in a either1.5 inch by 1.5 inch or 7 inch by 7 inch of a cathode corrugation by dipcoating at a substrate withdrawal rate of 1 inch/min. Gel film weretransferred to a furnace preheated to 70° C. just after gel filmdeposition, held for 3 hours and heated up to a temperature no more than600° C. for another 3 hours for complete crystallization and sintering.

After heating to 600° C., the film remained continuous, smooth and denseon the cathode side hardware body. FIG. 3 shows a SEM photograph of thesurface morphology of the LSC coated cathode side hardware.

The effect of thermal cycling on the integrity of the litiated NiO andLSC coated cathode side hardware sheets was also evaluated. Examinationof the post cycling specimens by optical microscopy and SEM revealedthat the coatings were thermally compatible with the hardware body orsubstrate, without detectable thermal stress induced cracks ormicrocracks.

The cross sectional SEM observations for a representative example ofcathode side hardware coated with lithiated NiO in accord with theinvention as compared with uncoated cathode side hardware is shown inFIGS. 5A-5D. As can be seen, the lithiated NiO coating of the invention(FIGS. 5A and 5B), had significantly reduced the thickness of thecorrosion scale, as compared to the uncoated hardware (FIGS. 5C and 5D).More particularly, the corrosion scales show a similar dual-layeredstructure whether or not sol-gel coated. However, for the coatedhardware, the outer oxide scale may have been denser to better protectthe inner Cr-rich scale. Consequently, the Cr-rich inner scale of thecoated sheets was denser and more protective to reduce the overallcorrosion rate.

FIG. 6A shows the out-of-cell electrical resistivity of cathode sidehardware in the form of a cathode current collector (“CCC”) having thelithiated NiO and LSC coatings of the invention. It also shows theout-of-cell electrical resistivity of a non-coated stainless steelcurrent collector. As can be seen, the coated CCCs of the invention havecomparable resistivities to the non-coated CCC. Additionally, as can beseen from FIG. 6B, the out-of-cell current collector-to-electroderesistivties (metal-to-metal) of the CCCs of the invention are alsocomparable to the current collector-to electrode resistivity of thenon-coated stainless steel CCC.

FIGS. 7 and 8 show the resistance lifegraphs of a fuel cells utilizingCCCs coated with the lithiated NiO and LSC coatings of the invention,respectively, as compared to fuel cells utilizing uncoated stainlesssteel CCCs. As can be appreciated from these graphs, an improvedresistance life is realized for the fuel cells using the CCCs with thecoatings of the invention.

FIG. 9 shows the corrosion thickness after fuel cell testing of CCCscoated with coatings of the invention as compared to the corrosionthickness after fuel cell testing of uncoated CCCs. The coated CCCs ofthe invention are seen to exhibit measurably less corrosion than theuncoated CCCs.

Finally, FIG. 10 illustrates the electrolyte loss in a molten carbonatefuel cell using CCCs coated with the coatings of the invention ascompared to the electrolyte loss in a molten carbonate fuel cell withuncoated CCCs. As can be seen, the electrolyte loss is considerably lessin the fuel cell using the coatings of the invention.

In all cases it is understood that the above-described arrangements aremerely illustrative of the many possible specific embodiments whichrepresent applications of the present invention. Numerous and variedother arrangements can be readily devised in accordance with theprinciples of the present invention without departing from the spiritand scope of the invention.

1. A carbonate fuel cell cathode current collector having a thin film coating of a conductive ceramic comprising one of LSC (La_(0.8)Sr_(0.2)CoO₃) and lithiated NiO (Li_(x)NiO, where x is 0.1 to 1).
 2. A carbonate fuel cell current collector in accordance with claim 1 wherein said coating is obtained by a sol-gel process.
 3. A carbonate fuel cell current collector in accordance with claim 2 wherein the thickness of said coating is between 0.5 μm and 5 μm.
 4. A carbonate fuel cell cathode bipolar plate having a thin film coating of a conductive ceramic comprising one of LSC (La_(0.8)Sr_(0.2)CoO₃) and lithiated NiO (Li_(x)NiO, where x is 0.1 to 1).
 5. A carbonate fuel cell cathode bipolar plate in accordance with claim 4 wherein said coating is obtained by a sol-gel process.
 6. A carbonate fuel cell cathode bipolar plate in accordance with claim 5 wherein the thickness of said coating is between 0.5 μm and 5 μm.
 7. A carbonate fuel cell comprising: a cathode; an anode; a matrix for storing carbonate electrolyte disposed between said anode and cathode; a cathode current collector situated adjacent said cathode; a bipolar plate situated adjacent said cathode current; and wherein at least one of said cathode current collector and said bipolar plate includes an electrically conductive ceramic comprising one of LSC (La_(0.8)Sr_(0.2)CoO₃) and lithiated NiO (Li_(x)NiO, where x is 0.1 to 1).
 8. A carbonate fuel cell in accordance with claim 7 wherein said coating is obtained by a sol-gel process.
 9. A carbonate fuel cell in accordance with claim 8 wherein the thickness of said coating is between 0.5 μm and 5 μm.
 10. A carbonate fuel cell in accordance with claim 7 wherein both said cathode current collector and said bipolar plate include an electrically conductive ceramic coating comprising one of LSC (La_(0.8)Sr_(0.2)CoO₃) and lithiated NiO (Li_(x)NiO, where x is 0.1 to 1).
 11. A carbonate fuel cell in accordance with claim 10 wherein said coatings are obtained by a sol-gel process.
 12. A carbonate fuel cell in accordance with claim 11 wherein the thickness of each of said coatings is between 0.5 μm and 5 μm.
 13. A method of making a carbonate fuel cell cathode current collector or a bipolar plate comprising the steps of: preparing a viscous solution having a viscosity in the range of 125 cp to 1000 cp at room temperature and containing one of: lithium and Ni cations; and La cations, Sr cations and Co cations; dipping a substrate into said viscous solution and withdrawing said substrate from said solution to form a coating of said viscous solution on said substrate; and heating said coated substrate after withdrawal from said viscous solution.
 14. A method according to claim 13, wherein: said viscosity of said viscous solution is in a range of 200 cp to 275 cp at room temperature.
 15. A method according to claim 14, wherein: said withdrawing of said substrate from said solution is at a rate of one inch/min.; and said step of heating is carried out at a temperature of 600° C. for a period of three hours.
 16. A method in accordance with claim 15, wherein: said substrate comprises stainless steel.
 17. A method in accordance with claim 16, wherein: said substrate is corrugated. 